This invention relates to compositions and processes for metal treatment prior to coating the metal surfaces with a protective layer of a phosphate compound. More particularly, novel colloidal alkyl benzene sulfonate salts compositions are provided which are useful for the treatment of ferrous type metals prior to applying protective coatings such as zinc or calcium phosphate compounds.
The art of forming protective layers on ferrous metals has been the subject of investigation for many years. It has long been known to apply coatings to metal, particularly to ferrous metals, for the purpose of inhibiting corrosion. Paint materials as protective coatings are common and it is known that by improving the adhesion of paint to the metal more durable protection is provided by the paint.
In efforts to provide improved adhesion of paint layers on ferrous metals it was discovered that a thin layer of zinc phosphate directly on the metal greatly increased the ability of paint to adhere as well as inhibit corrosion.
The phosphating art greatly improved when it was discovered that ferrous metal surfaces treated or contacted with a solution containing a small amount of titanium together with sodium phosphate prior to zinc phosphating provided a zinc phosphate layer on the metal which was more evenly distributed and in smaller particle size. This discovery was made by G. W. Jernstedt who was awarded several patents in the area including U.S. Pat. Nos. 2,310,239; 2,456,947; 2,462,196 and 2,490,062. It is believed that the solution of sodium phosphate and titanium "activate" the metal such that the metal is more readily coated with zinc phosphate in the following step. The titanium containing materials or activating compositions became known as "Jernstedt salts".
Typically, Jernstedt salts are prepared by first dissolving disodium phosphate in water and adding titanium as a soluble salt. The aqueous solution is heated within a limited temperature range of from about 60.degree. C. to about 85.degree. C. with mixing for about 10 hours. The solution is then evaporated to dryness at elevated temperatures and the dry material is used to prepare aqueous solutions for metal pretreatment. It is known that the temperature to which the initial titanium of sodium phosphate solution is heated prior to evaporation is critical. Such temperature can be varied within only narrow limits as higher temperatures result in degraded product and lower temperatures result in inactive product.
Because of the sensitivity of Jernstedt salts with respect to the temperature range employed in their preparation, attempts have been made to eliminate the need for any heat to form the dry solid activating compositions. One other reason for attempting to eliminate the heating step is to conserve energy. One attempt in reducing the requirement for heat in the preparation of Jernstedt salts is shown in U.S. Pat. No. 4,152,176 to Guhde. This patent describes a method for preparing a Jernstedt salt, which includes sodium tripolyphosphate as a cleaner for the metal, by the steps of preparing a mixture of water, sodium tripolyphosphate, disodium phosphate and a titanium-containing compound. This aqueous mixture is heated to a temperature of from 65.degree. C. to about 95.degree. C. and the solution is then added to solid disodium phosphate with mixing to obtain a solid titanium phosphate composition. The solid composition is then employed to prepare solutions for treating metal surfaces prior to aqueous phosphating in the typical manner. Although the final mixture with disodium phosphate is considered to provide a dry activating composition such compositions contain about 15% water. Such mixtures are then employed to prepare Jernstedt salt solutions in the usual manner.
Further attempts to improve upon Jernstedt salts is shown in U.S. Pat. No. 4,539,051 to Hacias. In the aforementioned patent the Jernstedt salt contains tetrasodium pyrophosphate in addition to the small amount of titanium and a sodium phosphate compound. It is claimed that by including the pyrophosphate, comparatively smaller amounts of the dry particuate salt are required in aqueous metal treating solution to activate the metal and thus provide superior phosphate coating.
The aqueous treatment bath temperature at which a metal surface is to be effectively treated has been known to be in the range of from about 49.degree. C. to about 64.degree. C. for one step cleaning and conditioning or activating. In an effort to reduce the required temperature of the bath, multi-component baths have been disclosed wherein the basic Jernstedt salt solution is employed together with other ingredients which perform adequate metal conditioning prior to the phosphating process. One such disclosure is found in U.S. Pat. No. 4,497,667 to Vashi. According to this patent the aqueous bath temperature is lowered to about 38.degree. C. while obtaining adequate conditioning. Surfactants, sequestrants, buffers such as alkali metal carbonates, silicates and other ingredients are employed to achieve the desired result.
In all of the known preparations of Jernstedt salts it is the conventional wisdom to incorporate in aqueous solution a titanium salt having some solubility in water together and a sodium phosphate compound followed by heating these ingredients together at a temperature within a narrow range. In most instances the aqueous solution is evaporated to dryness requiring close attention to avoid an inferior product.
While numerous attempts have been made to improve the performance of the titanium containing Jernstedt salt baths with respect to the activation of metal surfaces a common problem with all such salts has been the meticulous care with which the titanium containing material is prepared. Further, there is a need for more convenient preparation of an activating material and obvious benefits would inure should a less expensive material than titanium be found which will activate ferrous type metal as that obtained employing the traditional titanium containing Jernstedt salts.